Exposure apparatus and device manufacturing method using the same

ABSTRACT

An exposure apparatus for exposing, in an actual exposure process, an object with light through an optical system. The apparatus includes a transmission factor maintaining device for irradiating the optical system with light before the actual exposure process, so that a transmission factor of a portion of or the whole of the optical system is substantially unchanged as a result of the actual exposure process.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to an exposure apparatus and a devicemanufacturing method. More particularly, the invention is concerned withan exposure apparatus for use in an exposure process for the manufactureof devices such as a semiconductor devices (e.g., ICs or LSIs), imagepickup devices (e.g., CCDs), display devices (e.g., liquid crystalpanels) or sensors (e.g., magnetic heads), for example. In anotheraspect, the invention is concerned with a device manufacturing methodfor manufacturing devices such as described above.

An optical element such as a transparent plate, a lens or a prism usedin a projection optical system or an illumination optical system of aprojection exposure apparatus is formed on its bottom surface with anantireflection film (optical thin film). The provision of such ananti-reflection film is to efficiently direct light, from a lightsource, to a photosensitive substrate and also to prevent a flare or aghost from impinging on the photosensitive substrate. Since a lightsource of a projection optical system produces strong ultraviolet rays,intensive ultraviolet light is projected on the surface of an opticalelement of an illumination optical system or of a projection opticalsystem. Particularly, in a case wherein the light source comprises anexcimer laser which emits pulse light in the ultraviolet region, theenergy of ultraviolet light per unit time is very large. As a result,the spectral reflectivity characteristic of an antireflection film orthe absorptivity at various surfaces may slightly change, to cause achange in spectral transmissivity. Generally, an illumination opticalsystem and a projection optical system include optical elements havingsurfaces of a number of a few tens in total. Thus, even if the spectraltransmissivity change per one surface is small, it may cause a largespectral transmissivity change in total.

Spectral reflectivity of an anti-reflection film changed by irradiationof ultraviolet rays may change, if irradiation of ultraviolet light isstopped, to be restored to its original spectral reflectivitycharacteristic. Thus, the transmissivity of the illumination opticalsystem or projection optical system changes with the state of operationof the exposure apparatus. Such a phenomenon may attribute to watercontent or organic materials within the film from being disengaged byintensive ultraviolet rays entered into the film while, on the otherhand, in a state in which no ultraviolet light is projected, the watercontent or organic materials within the environment being absorbed bythe film.

Generally, the amount of exposure to be supplied to a photosensitivesubstrate may be controlled by receiving a portion of ultraviolet lightby the use of a photodetector disposed in the illumination opticalsystem and by detecting the light quantity upon the photosensitivesubstrate on the basis of the received light quantity and of the ratiobetween the received light quantity and the exposure amount, which maybe determined beforehand. Thus, if the transmissivity of an(illumination) optical system after such a photodetector and of theprojection optical system varies due to the phenomenon described above,the ratio between the light quantity as projected on the photodetectorand the light quantity supplied onto the photosensitive substrate maychange, causing an error in the detected value of the exposure amount.As a result, the photosensitive substrate cannot be exposed with acorrect exposure amount.

Additionally, there may be a case wherein a change in spectralreflectivity (spectral transmissivity) of an anti-reflection film causesa change in an illuminance distribution upon a photosensitive substrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an exposureapparatus and a device manufacturing method, by which a substrate can beexposed with a correct exposure amount.

In accordance with a first aspect of the present invention, there isprovided an exposure apparatus for illuminating a mask with anillumination optical system to expose a substrate in accordance with apattern of the mask, said apparatus comprising: transmissivitymaintaining means for maintaining, substantially constant, atransmissivity of a portion of or the whole of said illumination opticalsystem.

In accordance with a second aspect of the present invention, there isprovided an exposure apparatus for illuminating a mask with anillumination optical system and for projecting a pattern of the maskonto a substrate through a projection optical system, said apparatuscomprising: transmissivity maintaining means for maintaining,substantially constant, a transmissivity of a portion of or the whole ofa system provided by said illumination optical system and saidprojection optical system.

In accordance with a third aspect of the present invention, there isprovided an exposure apparatus for illuminating a mask with light from alight source and through an illumination optical system, and forprojecting a pattern of the mask onto a substrate through a projectionoptical system, said apparatus comprising: transmissivity maintainingmeans for projecting light, from the light source, to said illuminationoptical system and said projection optical system so as to maintain,substantially constant, a transmissivity of said illumination opticalsystem and said projection optical system. The light projection by saidtransmissivity maintaining means may be performed, basically, beforepractically exposing the substrate.

In the first to third aspects of the present invention, saidillumination optical system may include optical elements havinganti-reflection films formed on their light entrance and exit surfaces.

The illumination optical system may include a reflection mirror havingan intensified reflection film.

The illumination optical system and the projection optical system mayinclude optical elements having antireflection films formed on theirlight entrance and exit surfaces, wherein said optical elements mayinclude lens elements.

In the first to third aspects of the present invention, saidillumination optical system may include light dividing means fordividing light from a light source, and said apparatus may furthercomprise photoelectric converting means for receiving a portion of lightfrom said light source as provided by said light dividing means, andexposure amount control means for detecting and controlling the amountof exposure of the substrate on the basis of an output of saidphotoelectric converting means.

The transmissivity maintaining means may maintain, substantiallyconstant, the transmissivity of an optical system between said lightdividing means and the substrate.

The apparatus may further comprise transmissivity measuring means formeasuring transmissivity of an optical system between said lightdividing means and the substrate.

The transmissivity measuring means may include said photoelectricconverting means as well as second photoelectric converting means atleast having a light receiving portion provided on substrate holdingmeans, for holding the substrate and being movable, wherein saidphotoelectric converting means and said second photoelectric convertingmeans may operate to perform photoelectric conversion of lightsimpinging on them while said second photoelectric converting means maybe disposed opposed to a light exit surface of an optical system betweensaid light dividing means and the substrate, wherein a ratio of outputsof said photoelectric converting means and said second photoelectricconverting means may be calculated, and wherein the transmissivity maybe determined on the basis of the calculated output ratio.

In the first to third aspects of the present invention, saidtransmissivity maintaining means may maintain, substantively constant,the transmissivity of an optical system between said light dividingmeans and the substrate, by projecting light from the light source tothe optical system between said light dividing means and the substrate.

While predicting the transmissivity of an optical system between saidlight dividing means and the substrate on the basis of an output of saidphotoelectric converting means, light from the light source may beprojected to the optical system between said light dividing means andthe substrate, separately from a practical exposure operation, tothereby set the transmissivity between said light dividing means and thesubstrate at a desired value.

The amount of change of transmissivity of an optical system between saidlight dividing means and the substrate may be predicted on the basis ofan output of said photoelectric converting means and of timeinformation, wherein, when the amount of change of transmissivityexceeds a predetermined value, a predetermined quantity of light fromthe light source may be projected to the optical system between saidlight dividing means and the substrate, separately from a practicalexposure operation, to thereby set the transmissivity of the opticalsystem between said light dividing means and the substrate to a desiredvalue.

A predetermined quantity of light from the light source may beprojected, with a certain periodicity, to an optical system between saidlight dividing means and the substrate, to thereby maintain,substantially constant, the transmissivity of the optical system betweensaid light dividing means and the substrate, wherein said predeterminedperiodicity may be "once at a predetermined time per one day(twenty-four hours)" or "once at a predetermined time per two days(forty-eight hours)".

The transmissivity measuring means may measure the transmissivity of anoptical system between said light dividing means and the substrate,wherein, when the measured value is out of a predetermined range, apredetermined quantity of light from the light source may be projectedto the optical system between said light dividing means and thesubstrate, separately from a practical exposure operation, to therebyset the transmissivity of the optical system between said light dividingmeans and the substrate to a desired value.

In these cases, usually, before a practical (initial) exposureoperation, a predetermined quantity of light from the light source maybe projected to an optical system between said light dividing means andthe substrate to thereby set the transmissivity of the optical systembetween said light dividing means and the substrate to a desired value.

The optical system between said light dividing means and the substratemay include a projection optical system for projecting a pattern of themask onto the substrate, wherein said apparatus may further comprisecorrecting means for projecting light from the light source to anoptical system between said light dividing means and the substrate, tothereby compensate for a change in optical characteristic produced insaid projection optical system.

The correcting means may include predicting means for predicting anamount of change of the optical characteristic at a predetermined time,and adjusting means for adjusting said apparatus in accordance with thepredicted amount of change.

The optical characteristic may include a projection magnification ofsaid projection optical system, wherein said adjusting means may includeat least one of (i) moving means for moving one of a lens element ofsaid projection optical system and the mask in an optical axis directionof said projection optical system, (ii) pressure changing means forchanging a pressure of air between lenses of said projection opticalsystem, and (iii) wavelength changing means for changing a wavelength oflight from the light source.

The optical characteristic may include an imaging position of the maskpattern through said projection optical system, wherein said adjustingmeans may include at least one of (i) moving means for moving thesubstrate in an optical axis direction of said projection opticalsystem, (ii) pressure changing means for changing a pressure of airbetween lenses of said projection optical system, and (iii) wavelengthchanging means for changing a wavelength of light from the light source.

In the first to third aspects of the present invention, said lightapparatus may include an excimer laser as a light source for theexposure operation, wherein said excimer laser may comprise one of a KrFexcimer laser and an ArF excimer laser.

The illumination optical system may serve to define a slit-likeillumination region of a width smaller than the width of the wholepattern of the mask to be transferred, and wherein the mask and thesubstrate may be scanned relative to said illumination optical system ina direction perpendicular to the lengthwise direction of the slit-likeillumination region by which the whole mask pattern may be transferredto the substrate.

The illumination optical system may serve to define an illuminationregion of the same size as the whole pattern of the mask to the betransferred.

In accordance with a further aspect of the present invention, there isprovided a device manufacturing method which includes a process oftransferring a device pattern onto a substrate by use of an exposureapparatus such as described above.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exposure apparatus according to anembodiment of the present invention.

FIG. 2 is a graph for explaining the relation between an integratedlight quantity and transmissivity.

FIG. 3 is a graph for explaining the relation between the time withoutlight irradiation and the transmissivity.

FIG. 4 is a schematic view of an exposure apparatus according to anotherembodiment of the present invention.

FIG. 5 is a graph for explaining a variation of focus position of aprojection lens system.

FIG. 6 is a flow chart of semiconductor device manufacturing processes.

FIG. 7 is a flow chart for explaining a wafer process in the procedureof FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view of the structure of an exposure apparatusaccording to an embodiment of the present invention. Denoted at 1 is anexcimer laser which is a light source, and denoted at 2 is a beamshaping and incoherency transformation optical system for shaping laserlight from the laser light source 1 and for transforming it intoincoherent light. Denoted at 3 is an optical integrator, which comprisesa fly's eye lens, for example, which serves to define secondary lightsources. Denoted at 4 is a condenser lens for illuminating a portion ofa masking blade 6 about the opening thereof, with light from the opticalintegrator 3. Denoted at 5 is a beam splitter (or half mirror) forextracting a portion of the light from the optical integrator 3. Theextracted light is directed to a photodetector 10 (hereinafter "detector10"). On the basis of a photoelectric conversion output of the detector10, the amount of exposure of a photosensitive substrate (wafer) ismonitored.

The masking blade 6 comprises four light blocking plates which aremovable independently of each other. The opening of the masking blade 6is projected by an imaging lens 7 onto a reticle (mask) R, whereby onlya predetermined region on the reticle R is illuminated. Denoted at M1,M2 and M3 are deflection mirrors for deflecting the light path. Thesemirrors have intensified reflection films formed on their reflectionsurfaces. A circuit pattern formed on the reticle R is projected onto aphotosensitive substrate W through a projection optical system 8.Denoted at 9 is an X-Y stage for holding the photosensitive substratethereon and for moving it stepwise in the X and Y directions. Mounted onthe X-Y stage 9 is an illuminometer 11 for measuring the illuminanceupon a plane at the same level as the surface of the photosensitivesubstrate W during the exposure. Denoted at 12 is a calculating meansfor calculating an integrated exposure amount, from the light quantityas measured by the detector 10. Denoted at 13 is a main control for theprojection exposure apparatus.

FIG. 2 is a graph for explaining a variation of transmissivity of anoptical system, comprising the imaging lens 7 and the projection opticalsystem 8, to changes in integrated (irradiation) light quantity, inresponse to irradiation with light from the laser 1. The graph shows aresult of measurement having been made beforehand. For the measurement,the illuminometer 11 is moved by the X-Y stage 9 into the irradiationregion under the projection optical system 8, and the output of thedetector 10 per unit time and the output of the illuminometer 11 perunit time are detected, wherein, with respect to an integrated value asmeasured by the detector 10, the ratio of these outputs is detected. Asseen in FIG. 2, the transmissivity of the system increases with theirradiation, and it is saturated with an integrated light quantityhigher than a certain level.

FIG. 3 is a graph for explaining a variation, with respect to time, ofthe transmissivity of the system comprising the imaging lens 7 and theprojection optical system 8, in a state wherein the apparatus is leftwithout being irradiated with light. For the measurement, theilluminometer 11 is moved to a position similar to that in the FIG. 2example, and the transmissivity is measured by use of the outputs of thedetectors 10 and 11 while lighting the light source 1 periodically onlyfor a certain time. It is seen from FIG. 3 that, if the apparatus isleft without being irradiated with light, the transmissivity of thesystem gradually decreases with an elapse of time, and it becomesconstant at a certain level.

On the basis of the results as shown in FIGS. 2 and 3, the main control13 of the projection exposure apparatus of the FIG. 1 embodimentoperates to predict, when the apparatus is under operation, the value oftransmissivity of the system comprising the condensing lens 7 and theprojection optical system 8, in accordance with the integrated lightquantity per unit time as monitored through the detector 10 and withvalues of the aperture area of the masking blade 6 and thetransmissivity of the reticle R. If the apparatus is left without lightirradiation, the main control predicts the transmissivity of the system,comprising the imaging lens 7 and the projection optical system 8, fromthe transmissivity just before the apparatus is left and from the timein which the apparatus has been left. If, during non-operation, thevalue of transmissivity of the system comprising the imaging lens 7 andthe projection optical system 8 becomes lower than a predetermined(threshold) value, the main control 13 applies a signal to the laser 1,separately from a practical exposure operation, to cause the laser 1 toemit light so that the light is projected to the system, until thetransmissivity of the system comprising the imaging lens 7 and theprojection optical system 8 increases to the predetermined level.

Here, if the value of the integrated light quantity as projected to thesystem comprising the imaging lens 7 and the projection optical system8, being calculated by the calculating means 12 on the basis of theoutput of the detector 10, reaches a desired level, the main control 13stops light emission of the laser 1.

Similarly, before a start of operation of the exposure apparatus, thelaser 1 is energized to emit light to irradiate the system with thelight, so that the value of transmissivity of the system increases (ordecreases) to a predetermined level. This applies similarly to otherembodiments, to be described below.

In place of determining the integrated light quantity by monitoring theactual light quantity by use of the detector 10, it may be controlled inaccordance with the number of emitted pulses or the time of irradiationwherein the light source comprises a pulse laser, as in the presentembodiment. When the light source 1 comprises a lamp, a shutter (notshown) may be opened for a predetermined time.

The excimer laser 1 may comprise a KrF excimer laser or an ArF excimerlaser, wherein the half width of a spectral line is band-narrowed to 3pm or less. While the projection optical system 8 of this embodimentcomprises a lens system constituted only by SiO₂, a lens systemcomprising a combination of SiO₂ lenses and CaF₂ lenses may be used.Alternatively, the projection optical system 8 may comprise catadiopticshaving a lens and a concave mirror. Further, for an improved resolvingpower and enhanced transmissivity, the projection optical system mayinclude an aspherical surface lens or a diffractive optical element suchas binary optics or a kinoform, for example. This applies to otherembodiments to be described later.

The beam shaping and incoherency transformation optical system 2 has aknown structure such as disclosed in Japanese Published Laid-Open PatentApplication No. 47639/1993, for example, and a description of details ofit will be omitted here.

In FIG. 2, the saturated transmissivity is at a level higher, by a fewpercent, than the transmissivity before a start of light irradiation ofthe optical system 7 and 8, and it is seen that it took several tens ofhours until the saturation.

In this embodiment, for a correct exposure amount control, light isprojected appropriately to the optical system 7 and 8 from the exposurelight source, so that the transmissivity of the optical system 7 and 8is maintained in a certain range of ±1% about the value oftransmissivity as saturated. Also, by doing such transmissivity control,non-uniformness of illuminance upon the reticle R or photosensitivesubstrate W can be kept small. This applies to other embodiments to bedescribed later. The range described above may of course be changed asdesired or in accordance with the type of the apparatus.

While the embodiment has been described with reference to an examplewherein the transmissivity of the system comprising the imaging opticalsystem 7 and the projection optical system 8 increases in response tolight irradiation and it decreases in response to stopping of the lightirradiation, there may be an inverse phenomenon, depending on thecharacteristics of an anti-reflection film formed on the light entranceor exit surfaces of optical elements, such as the lens, prism or plate,constituting the illumination optical system 14 or projection opticalsystem 8. The present invention is also applicable to such cases. Theanti-reflection film of this embodiment comprises alternate layers ofAl₂ O₃ and SiO₂, while the intensified reflection film formed on eachmirror comprises alternate layers of HaO₂ and SiO₂.

The time in which a variation of transmissivity of the optical system 7and 8, as they are left without being irradiated, exceeds theabove-described predetermined range can be determined from the resultsshown in FIGS. 2 and 3. Thus, the main control 13 may energize the laser1 during an elapse of this time to emit light until the integrated lightquantity at the output of the detector 10 reaches a predetermined level,such that the transmissivity of the optical system 7 and 8 may bemaintained within that range.

In a case wherein the apparatus is continuously held out of operationfor any reason and the transmissivity goes beyond the predeterminedrange, an integrated light quantity which can meet such an occasion maybe determined beforehand and the main control 13 may operate to lightthe laser 1 automatically at the moment of which the apparatus isrestored and before a start of an actual exposure operation, so thatlight of a necessary quantity may be applied to the optical system 7 and8. The integrated light quantity may be controlled on the basis of thenumber of total pulses projected.

In this embodiment, separately from the actual exposure operation, thelaser 1 is excited to emit light at a predetermined time moment, onceper day or once per two days, for example.

In the exposure apparatus of FIG. 1, the illuminometer 11 is moved bythe X-Y stage 9 into the irradiation region of the projection opticalsystem 8, and while engaging the laser 1 to emit light, thetransmissivity of the optical system 7 and 8 is measured on the basis ofthe output ratio between the detector 10 and illuminometer 11. If theresult of the measured transmissivity is out of the predetermined rangeas described, the light source 1 is lighted in response to a signal fromthe main control 13. Even during the light being emitted, the outputratio between the detector 10 and the illuminometer 11 may be measuredto determine the transmissivity of the optical system 7 and 8, so thatthe light emission of the laser 1 may be stopped by the main control 13when the transmissivity of the optical system 7 and 8 reaches apredetermined level within the predetermined range.

FIG. 4 is a schematic view of the structure of an exposure apparatusaccording to another embodiment of the present invention. In FIG. 4,reference numerals similar to those of FIG. 1 are assigned tocorresponding elements, and a duplicate explanation of them will beomitted, for simplicity.

Denoted at 21 is a field lens which is a component of the projectionoptical system. It is held by a field lens driving mechanism 22, and itcan be moved in response to a signal from the main control 13, in theoptical axis direction of the optical system 8, to correct or compensatefor a change in projection magnification of the projection opticalsystem 8, or to set it at a desired value. Denoted at 23 and 24 is anautofocus detection system for detecting the height (level) of thesurface of a photosensitive substrate W with respect to the optical axisdirection. It comprises an illumination system 23 for illuminating thephotosensitive substrate W and a light receiving system 24 for receivinglight reflected by the surface of the photosensitive substrate W. Themain control 13 detects the position of the photosensitive substrate Win accordance with the position of light reception and actuates a Zstage, mounted on the X-Y stage 9, in accordance with the positiondetection so that the surface of the photosensitive substrate W isbrought into registration with the best focus plane of the projectionoptical system 8. Denoted at 26 is optical characteristics predictingmeans for calculating the amount of change of optical characteristics ofthe projection optical system, such as focus position or projectionmagnification, for example.

When light from the laser 1 enters the projection optical system 8,lenses of the projection optical system 8 absorb a portion of the lightand their temperature rises slightly. The optical characteristicpredicting means 26 predicts a change in optical characteristic of theprojection optical system 8, from the result of measurement of the lightquantity by the detector 10, the aperture area of the masking blade 6and the value of transmissivity of the reticle R.

FIG. 5 shows predicted values of a change in focus position as the lightenters the projection optical system 8. While the illustrated graphconcerns the focus position, changes of projection magnification can bepredicted in a similar manner.

On the basis of a prediction by the optical characteristic predictingmeans 26, the main control 13 supplies a driving signal to the fieldlens driving mechanism 22, with regard to the magnification, to move thefield lens at a predetermined position along the optical axis direction,to thereby compensate for the change of projection magnification of theprojection optical system 8. With regard to changes in focus position,the main control 13 operates to move the Z stage 25 in the optical axisdirection so that the surface of the photosensitive substrate W isbrought into alignment with the focus plane of the projection opticalsystem 8, whose position is variable with irradiation with light, whiletaking into account a predetermined offset to be added to the heightinformation as measured by the light receiving system 24.

The main control 13 predicts the amount of change of opticalcharacteristic of the projection optical system 8 in association withthe optical characteristic predicting means 26, and, in accordance withthe result of it, the main control supplies an offset as described,corresponding to the predicted value, to the autofocusing systemincluding the sensor (23 and 24). The main control 13 takes thereintoparameters for calculation of the amount of change, wherein theparameters may include the time period t of irradiation of the opticalsystem 7 and 8 with light, the time t' between irradiation periods, theoutput of the detector 10, the aperture area of the masking blade 6, thequantity QD of light projected to the optical system 8 as can becalculated from the transmissivity of the reticle R, or a coefficient Dapeculiar to the reticle R used.

From these parameters and coefficients originally set in the apparatus,the optical characteristic predicting means 26 predicts changes ofoptical characteristics during repetition of light projection. Acalculation therefor will be explained with reference to an example of achange ΔF of the focus position of the projection optical system 8. Thiscalculation uses first-order equations.

    ΔF=ΔF1+ΔF2

    ΔF1=SF·QD·Da·DT

    ΔF2=ΔF'·exp(-k.sub.F -t)

wherein SF is a proportional constant, QD is a parameter correspondingto the total quantity of light passing through the circuit pattern, Dais a correction coefficient peculiar to the reticle R used, DT is theproportion of the time period in which, during a unit time to be usedfor calculation, the light projection has been made, and k_(F) is aparameter which represents thermal conduction of the optical elements ofthe projection optical system 4. ΔF' is the amount of change of thefocus position of the projection optical system 8, as calculated in thepreceding unit time. ΔF1 is the amount of change of focus position perunit time, due to heat absorption of the projection optical system 8,and ΔF2 is the amount of change of the focus plane of the projectionoptical system per unit time, due to heat discharge therefrom. ΔF2 maybe expressed in terms of linear coupling of plural terms.

A calculation by the optical characteristic predicting means 26 is maderepeatedly at unit time periods, and the amount of change of the focusposition of the projection optical system 8 detected by calculationvaries along a curve having an envelope represented by a function of anatural logarithm such as shown in FIG. 5.

Correction of the projection magnification of the projection opticalsystem 8 may be provided, other than by the driving means 22 for movingthe field lens 21. For example, at least one of (i) driving means formoving the reticle R in the optical axis direction, in a case whereinthe projection optical system 8 is not telecentric on the light entranceside, (ii) pressure changing means for changing the pressure of a space(closed space) between a pair of lenses of the projection optical system8, and (iii) wavelength changing means for changing the emissionwavelength of the excimer laser (light source) 1, may be used.

Correction of the imaging position of the pattern of the reticle Rthrough the projection optical system 8, that is, of the focus position,may be provided, other than by the moving means for moving thephotosensitive substrate W in the optical axis direction of theprojection optical system 8. For example, the correction may beperformed by using at least one of (i) pressure changing means 27 forchanging the pressure of a spaced (closed space) between a pair oflenses of the projection optical system 8, and (ii) wavelength changingmeans (provided by main control 13) for changing the emission wavelengthof the excimer laser 1.

While the embodiments of FIGS. 1-5 have been described with reference toan example of a projection type exposure apparatus, the presentinvention is applicable also to any exposure apparatus, such as aproximity type or contact type exposure apparatus, including lenses withlight entrance and exit surfaces formed with anti-reflection filmswherein similar problems are involved.

Further, while in the embodiments of FIGS. 1-5, the integrated lightquantity is measured during the exposure process by using the beamsplitter 5 and the detector 10, and the exposure amount control to thephotosensitive substrate W is made on the basis of the measurement, theexposure amount control may be made by irradiating the photosensitivesubstrate with a determined number of light pulses from the excimerlaser 1.

Further, in the embodiments of FIGS. 1-5, the transmissivity of theoptical system is maintained constant. This is because thephotosensitive substrate can be exposed with a correct exposure amount,provided that at least the transmissivity of the optical system 7 and 8is substantially constant. The structure may of course be arranged sothat the transmissivity of a system comprising the illumination opticalsystem 14 and the projection optical system 8 is controlled andmaintained substantially constant. Practically, on that occasion, thecharacteristic of a transmissivity change of such a system as a whole issimilar to that shown in FIG. 2 or 3. Thus, in the embodiments of FIGS.1-5, the transmissivity of the system comprising the illuminationoptical system 14 and the projection optical system 8 is maintainedsubstantially constant.

Next, an embodiment of a device manufacturing method which uses anexposure apparatus such as described above, will be explained.

FIG. 6 is a flow chart of a procedure for the manufacture ofmicrodevices such as semiconductor chips (e.g., ICs or LSIs), liquidcrystal panels or CCDS, for example. Step 1 is a design process fordesigning a circuit of a semiconductor device. Step 2 is a process formaking a mask on the basis of the circuit pattern design. Step 3 is aprocess for preparing a wafer by using a material such as silicon. Step4 is a wafer process which is called a pre-process wherein, by using theso prepared mask and wafer, circuits are practically formed on the waferthrough lithography. Step 5 subsequent to this is an assembling stepwhich is called a post-process wherein the wafer having been processedby step 4 is formed into semiconductor chips. This step includes anassembling (dicing and bonding) process and a packaging (chip sealing)process. Step 6 is an inspection step wherein an operation check, adurability check and so on for the semiconductor devices provided bystep 5, are carried out. With these processes, semiconductor devices arecompleted and they are shipped (step 7).

FIG. 7 is a flow chart showing details of the wafer process. Step 11 isan oxidation process for oxidizing the surface of a wafer. Step 12 is aCVD process for forming an insulating film on the wafer surface. Step 13is an electrode forming process for forming electrodes upon the wafer byvapor deposition. Step 14 is an ion implanting process for implantingions to the wafer. Step 15 is a resist process for applying a resist(photosensitive material) to the wafer. Step 16 is an exposure processfor printing, by exposure, the circuit pattern of the mask on the waferthrough the exposure apparatus described above. Step 17 is a developingprocess for developing the exposed wafer. Step 18 is an etching processfor removing portions other than the developed resist image. Step 19 isa resist separation process for separating the resist material remainingon the wafer after being subjected to the etching process. By repeatingthese processes, circuit patterns are superposedly formed on the wafer.

With these processes, high density microdevices can be manufactured.

In accordance with the embodiments of the present invention as describedhereinbefore, the transmissivity of an illumination optical system or aprojection optical system can be maintained substantially constant,during the exposure process of a photosensitive substrate. Thus, aphotosensitive substrate such as a wafer can be exposed with a correctexposure amount. This is particularly effective for an exposureapparatus or a device manufacturing method which uses, as a lightsource, an ultraviolet laser such as a KrF excimer laser or an ArFexcimer laser which emits a large intensity pulse light. Also, there isan additional advantage that the non-uniformness of illuminance on thesurface to be exposed can be made small.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An exposure apparatus for exposing, in an actualexposure process, an object with light through an optical system, saidapparatus comprising:irradiating means for irradiating the opticalsystem with light prior to said apparatus performing an actual exposureprocess; transmission factor maintaining means for controlling theirradiating of the optical system by said irradiating means, to maintaina transmission factor of at least a portion of the optical system withina range of ±1% about a value of the transmission factor at saturation ofthe optical system; and exposure means for exposing, in an actualexposure process, an object with light through the optical system.
 2. Anexposure apparatus according to claim 1, wherein the optical systemcomprises an illumination optical system for illuminating a pattern of areticle with light from a light source, and a projection optical systemfor projecting an image of the pattern of the reticle onto a substratewith light from the pattern.
 3. An apparatus according to claim 2,wherein both of said illumination optical system and said projectionoptical system include optical elements having anti-reflection filmsformed on their light entrance and exit surfaces.
 4. An apparatusaccording to claim 3, wherein said optical elements include lenselements.
 5. An exposure apparatus according to claim 2, wherein saidtransmission factor maintaining means controls said irradiating means toproject light to the optical system before the actual exposure process,to maintain the transmission factor of the whole optical systemcomprising said illumination optical system and said projection opticalsystem substantially unchanged as a result of the actual exposureprocess.
 6. An apparatus according to claim 5, wherein said illuminationoptical system includes a reflection mirror having an intensifiedreflection film.
 7. An apparatus according to claim 5, wherein saidillumination optical system and said projection optical system includeoptical elements having antireflection films formed on their lightentrance and exit surfaces.
 8. An apparatus according to claim 5,wherein said optical elements include lens elements.
 9. An apparatusaccording to claim 5, wherein said illumination optical system includeslight dividing means for dividing light from a light source, and whereinsaid apparatus further comprises photoelectric converting means forreceiving a portion of light from said light source as provided by saidlight dividing means, and exposure amount control means for detectingand controlling an amount of exposure of the substrate on a basis of anoutput of said photoelectric converting means.
 10. An apparatusaccording to claim 9, wherein said apparatus further comprisestransmissivity measuring means for measuring transmissivity of anoptical system between said light dividing means and the substrate. 11.An apparatus according to claim 10, wherein said transmissivitymeasuring means includes said photoelectric converting means as well assecond photoelectric converting means at least having a light receivingportion provided on substrate holding means, for holding the substrateand being movable, wherein said photoelectric converting means and saidsecond photoelectric converting means perform photoelectric conversionof lights impinging on them while said second photoelectric convertingmeans is disposed opposed to a light exit surface of an optical systembetween said light dividing means and the substrate, wherein a ratio ofoutputs of said photoelectric converting means and said secondphotoelectric converting means is calculated, and wherein thetransmissivity is determined on a basis of the calculated output ratio.12. An apparatus according to claim 9, wherein, while predicting thetransmissivity of an optical system between said light dividing meansand the substrate on the basis of an output of said photoelectricconverting means, light from the light source is projected to theoptical system between said light dividing means and the substrate,separately from a practical exposure operation, to thereby set thetransmissivity between said light dividing means and the substrate at adesired value.
 13. An apparatus according to claim 9, wherein an amountof change of transmissivity of an optical system between said lightdividing means and the substrate is predicted on the basis of an outputof said photoelectric converting means and of time information, wherein,when the amount of change of transmissivity exceeds a predeterminedvalue, a predetermined quantity of light from the light source isprojected to the optical system between said light dividing means andthe substrate, separately from a practical exposure operation, tothereby set the transmissivity of the optical system between said lightdividing means and the substrate to a desired value.
 14. An apparatusaccording to claim 9, wherein a predetermined quantity of light from thelight source is projected, with a certain periodicity, to an opticalsystem between said light dividing means and the substrate, to therebymaintain, substantially constant, the transmissivity of the opticalsystem between said light dividing means and the substrate.
 15. Anapparatus according to claim 14, wherein said predetermined periodicitycorresponds to a period of one day or two days.
 16. An apparatusaccording to claim 9, wherein said transmissivity measuring meansmeasures the transmissivity of an optical system between said lightdividing means and the substrate, wherein, when the measured value isout of a predetermined range, a predetermined quantity of light from thelight source is projected to the optical system between said lightdividing means and the substrate, separately from a practical exposureoperation, to thereby set the transmissivity of the optical systembetween said light dividing means and the substrate to a desired value.17. An apparatus according to claim 9, wherein, before a practicalexposure operation, a predetermined quantity of light from the lightsource is projected to an optical system between said light dividingmeans and the substrate to thereby set the transmissivity of the opticalsystem between said light dividing means and the substrate to a desiredvalue.
 18. An apparatus according to claim 9, wherein said apparatusfurther comprises correcting means for correcting a change in opticalcharacteristic produced in said projection optical system due toirradiation of said projection optical system with light from the lightsource.
 19. An apparatus according to claim 18, wherein said correctingmeans includes predicting means for predicting an amount of change ofthe optical characteristic at a predetermined time, and adjusting meansfor adjusting said apparatus in accordance with the predicted amount ofchange.
 20. An apparatus according to claim 19, wherein the opticalcharacteristic includes a projection magnification of said projectionoptical system, and wherein said adjusting means includes at least oneof (i) moving means for moving one of a lens element of said projectionoptical system and the mask in an optical axis direction of saidprojection optical system, (ii) pressure changing means for changing apressure of air between lenses of said projection optical system, and(iii) wavelength changing means for changing wavelength of light fromthe light source.
 21. An apparatus according to claim 19, wherein theoptical characteristic includes an imaging position of the mask patternthrough said projection optical system, and wherein said adjusting meansincludes at least one of (i) moving means for moving the substrate in anoptical axis direction of said projection optical system, (ii) pressurechanging means for changing a pressure of air between lenses of saidprojection optical system, and (iii) wavelength changing means forchanging wavelength of light from the light source.
 22. An apparatusaccording to claim 1, wherein said illumination optical system includesoptical elements having anti-reflection films formed on their lightentrance and exit surfaces.
 23. An apparatus according to claim 22,wherein said optical elements include lens elements.
 24. An apparatusaccording to claim 1, wherein said illumination optical system includesa reflection mirror having an intensified reflection film.
 25. Anapparatus according to claim 1, wherein said illumination optical systemincludes light dividing means for dividing light from a light source,and wherein said apparatus further comprises photoelectric convertingmeans for receiving a portion of light from said light source asprovided by said light dividing means, and exposure amount control meansfor detecting and controlling the amount of exposure of the substrate ona basis of an output of said photoelectric converting means.
 26. Anapparatus according to claim 25, wherein said transmission factormaintaining means maintains, substantially constant, the transmissivityof an optical system between said light dividing means and thesubstrate.
 27. An apparatus according to claim 26, wherein saidtransmission factor maintaining means maintains, substantially constant,the transmissivity of an optical system between said light dividingmeans and the substrate, by projecting light from the light source tothe optical system between said light dividing means and the substrate.28. An apparatus according to claim 27, wherein, while predicting thetransmissivity of an optical system between said light dividing meansand the substrate on the basis of an output of said photoelectricconverting means, light from the light source is projected to theoptical system between said light dividing means and the substrate,separately from a practical exposure operation, to thereby set thetransmissivity between said light dividing means and the substrate at adesired value.
 29. An apparatus according to claim 27, wherein an amountof change of transmissivity of an optical system between said lightdividing means and the substrate is predicted on the basis of an outputof said photoelectric converting means and of time information, wherein,when an amount of change of transmissivity exceeds a predeterminedvalue, a predetermined quantity of light from the light source isprojected to the optical system between said light dividing means andthe substrate, separately from a practical exposure operation, tothereby set the transmissivity of the optical system between said lightdividing means and the substrate to a desired value.
 30. An apparatusaccording to claim 27, wherein a predetermined quantity of light fromthe light source is projected, with a certain periodicity, to an opticalsystem between said light dividing means and the substrate, to therebymaintain, substantially constant, the transmissivity of the opticalsystem between said light dividing means and the substrate.
 31. Anapparatus according to claim 30, wherein said predetermined periodicitycorresponds to a period of one day or two days.
 32. An apparatusaccording to claim 27, wherein said transmissivity measuring meansmeasures the transmissivity of an optical system between said lightdividing means and the substrate, wherein, when the measured value isout of a predetermined range, a predetermined quantity of light from thelight source is projected to the optical system between said lightdividing means and the substrate, separately from a practical exposureoperation, to thereby set the transmissivity of the optical systembetween said light dividing means and the substrate to a desired value.33. An apparatus according to claim 27, wherein, before an actualexposure operation, a predetermined quantity of light from the lightsource is projected to an optical system between said light dividingmeans and the substrate to thereby set the transmissivity of the opticalsystem between said light dividing means and the substrate to a desiredvalue.
 34. An apparatus according to claim 27, wherein the opticalsystem between said light dividing means and the substrate includes aprojection optical system for projecting a pattern of the mask onto thesubstrate.
 35. An apparatus according to claim 34, wherein saidapparatus further comprises correcting means for projecting light fromthe light source to an optical system between said light dividing meansand the substrate, to thereby compensate for a change in opticalcharacteristic produced in said projection optical system.
 36. Anapparatus according to claim 35, wherein said correcting means includespredicting means for predicting an amount of change of the opticalcharacteristic at a predetermined time, and adjusting means foradjusting said apparatus in accordance with the predicted amount ofchange.
 37. An apparatus according to claim 36, wherein the opticalcharacteristic includes a projection magnification of said projectionoptical system, and wherein said adjusting means includes at least oneof (i) moving means for moving one of a lens element of said projectionoptical system and the mask in an optical axis direction of saidprojection optical system, (ii) pressure changing means for changing apressure of air between lenses of said projection optical system, and(iii) wavelength changing means for changing a wavelength of light fromthe light source.
 38. An apparatus according to claim 36, wherein theoptical characteristic includes an imaging position of the mask patternthrough said projection optical system, and wherein said adjusting meansincludes at least one of (i) moving means for moving the substrate in anoptical axis direction of said projection optical system, (ii) pressurechanging means for changing a pressure of air between lenses of saidprojection optical system, and (iii) wavelength changing a means forchanging wavelength of light from the light source.
 39. An apparatusaccording to claim 25, wherein said apparatus further comprisestransmissivity measuring means for measuring transmissivity of anoptical system between said light dividing means and the substrate. 40.An apparatus according to claim 39, wherein said transmissivitymeasuring means includes said photoelectric converting means as well assecond photoelectric converting means at least having a light receivingportion provided on substrate holding means, for holding the substrateand being movable, wherein said photoelectric converting means and saidsecond photoelectric converting means perform photoelectric conversionof lights impinging on them while said second photoelectric convertingmeans is disposed opposed to a light exit surface of an optical systembetween said light dividing means and the substrate, wherein a ratio ofoutputs of said photoelectric converting means and said secondphotoelectric converting means is calculated, and wherein thetransmissivity is determined on the basis of the calculated outputratio.
 41. An apparatus according to claim 1, wherein said apparatuscomprises an excimer laser as a light source for the exposure operation.42. An apparatus according to claim 41, wherein said excimer lasercomprises one of KrF excimer laser and ArF excimer laser.
 43. Anapparatus according to claim 1, wherein said illumination optical systemdefines a slit-like illumination region having a width smaller than thewidth of the whole pattern of the mask to be transferred, and whereinthe mask and the substrate are scanned relative to said illuminationoptical system in a direction perpendicular to the lengthwise directionof the slit-like illumination region by which the whole mask pattern istransferred to the substrate.
 44. An apparatus according to claim 1,wherein said illumination optical system defines an illumination regionof the same size as the whole pattern of the mask to be transferred. 45.A device manufacturing method for exposing, in an actual exposureprocess, an object with light through an optical system, said methodcomprising:irradiating the optical system with light before the actualexposure process; controlling, with transmission factor maintainingmeans, the irradiating of the optical system in said irradiating step,to maintain a transmission factor of at least a portion of the opticalsystem within a range of ±1% about a value of the transmission factor atsaturation of the optical system; and exposing, in an actual exposureprocess, an object with light through the optical system, to transfer adevice pattern onto a substrate to manufacture a device.
 46. A methodaccording to claim 45, wherein the optical system comprises anillumination optical system for illuminating a pattern of a reticle withlight from a light source, and a projection optical system forprojecting an image of the pattern of the reticle onto a substrate withlight from the pattern.
 47. A method according to claim 46, wherein theillumination optical system includes optical elements havinganti-reflection films formed on their light entrance and exit surfaces.48. A method according to claim 47, wherein the illumination opticalsystem includes a reflection mirror having an intensified reflectionfilm.
 49. A method according to claim 46, wherein both of theillumination optical system and the projection optical system includeoptical elements having anti-reflection films formed on their lightentrance and exit surfaces.
 50. A method according to claim 47, whereinthe optical elements include lens elements.
 51. A method apparatusaccording to claim 46, further comprising projecting, with thetransmission factor maintaining means, light to the optical systembefore the actual exposure process, so that a transmission factor of thewhole optical system comprising the illumination optical system and theprojection optical system is substantially unchanged as a result of theactual exposure process.
 52. A method according to claim 51, wherein theillumination optical system includes a reflection mirror having anintensified reflection film.
 53. A method according to claim 51, whereinthe illumination optical system and the projection optical systeminclude optical elements having anti-reflection films formed on theirlight entrance and exit surfaces.
 54. A method according to claim 53,wherein the optical elements include lens elements.